Droplet actuators are used to conduct a wide variety of droplet operations. A droplet actuator typically includes a substrate associated with droplet operation electrodes, which are arranged in a manner which permits certain droplet operations to be performed on a surface of the substrate. In some cases, the droplet actuator also includes a top plate separated from the surface of the substrate in a manner which creates a gap in which the droplet operations may be performed. Typically the droplets are at least partially surrounded by filler fluid that is immiscible with the droplets. The surface of the substrate and other surfaces exposed to the filler fluid and/or the droplet are typically made from or coated with a material which is hydrophobic relative to the droplets being manipulated.
Referring to the applicants unpublished patent applications, which are cited herein, certain kinds of droplets may be subjected to various droplet operations on a droplet actuator to conduct certain kinds of droplet-based protocols. Where the droplet-based protocols involve detection of an analyte that is present in an initial sample in scarce amounts, there is a need for approaches to concentrating the analyte for analysis in the very small droplets that are used on the droplet actuator.